Nickel-Chromium-Iron-Molybdenum Corrosion Resistant Alloy and Article of Manufacture and Method of Manufacturing Thereof

ABSTRACT

A solid-solution nickel-based alloy for use in sour gas and oil environments, including, in percent by weight: chromium: min. of 21.0 and max. of 24.0%; iron: min. of 17.0 and max. of 21.0%; molybdenum: min. of 6.5 and max. of 8.0%; copper: min. of 1.0 and max. of 2.5%; tungsten: min. of 0.1 and max. of 1.5%; sol. nitrogen: min. of 0.08 and max. of 0.20%; manganese: max. of 4.0%; silicon: max. of 1.0%; carbon: max of. 0.015%; aluminum: max of 0.5%; and a total amount of niobium, titanium, vanadium, tantalum, and zirconium: max of 0.45%; the balance being nickel and incidental impurities, along with a method of manufacturing an article from the alloy, and an article of manufacture formed from the alloy.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/014,803 filed Jun. 20, 2014, the disclosure of which is herebyincorporated by reference for all purposes in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to solid solutionstrengthened nickel-chromium-iron-molybdenum corrosion resistant alloyssuitable for use in sour gas and oil environments.

2. Description of Related Art

Nickel alloys generally have the ability to withstand a wide variety ofsevere operating conditions involving corrosive environments, hightemperatures, and high stresses. From commercially pure nickel tocomplex alloys containing as many as twelve or more alloying elements,nickel alloys are used for a wide variety of applications, including,for example: chemical and petrochemical industries; pulp and papermills; aircraft gas turbines; steam turbine power plants, reciprocatingengines; metal processing; medical applications; space vehicles,heat-treating equipment; nuclear power systems; pollution controlequipment; metals processing mills; coal gasification and liquefactionsystems; automotive industries; and oil and gas industries.

Deep wells are important for the future of oil and gas exploration. Deepwells are generally categorized as being either sweet or sour, withsweet wells being mildly corrosive and sour wells being highlycorrosive, containing combinations of corrosive agents, such as hydrogensulfide, carbon dioxide, chlorides, and free sulfur. The corrosiveconditions of sour wells are compounded by high temperatures and highpressures. Thus, for sour oil and gas environments, materials areselected to meet stringent criteria for corrosion resistance whileachieving excellent mechanical properties.

Nickel-chromium-iron-molybdenum alloys offer advantages over othermaterials used in oil and gas industries, including high strength, hightoughness, and excellent corrosion resistance.Nickel-chromium-iron-molybdenum alloys include solid solution alloys andprecipitation hardenable alloys. Solid solution alloys generally obtaintheir strength through solid solution strengthening and cold working.Precipitation hardenable alloys are generally used for heavier crosssections, which cannot be easily strengthened by cold working, or areused at higher temperatures, at which the effect of cold working is notsustained. The precipitation hardenable alloys primarily obtain theirstrength as a result of precipitation of secondary phases within thematrix of the alloy. Precipitation hardening is often the result of theprecipitation via heat treatment of gamma prime or gamma double primephases. Formation of carbides and nitrides may also strengthenprecipitation hardenable alloys. However, precipitation hardenablealloys have disadvantages compared to solid solution alloys. Therefore,the use of precipitation hardenable alloys is generally limited toapplications for which solid solution alloys are not desirable.

INCOLOY® alloy 028 (UNS N08028/W. Nr. 1.4563), a nickel-iron-chromiumalloy with additions of molybdenum and copper, is especially resistantto sulfuric and phosphoric acid and is commonly used for chemicalprocessing, pollution control equipment, oil and gas well piping,nuclear fuel reprocessing, acid production, and pickling equipment. Thechemical composition of INCOLOY® alloy 028 is, by weight percent: Ni:30-32; Fe: 22 min.; Cr: 26-28; Mo: 3-4; Cu: 0.60-1.40; C: 0.02 max; Mn:2 max; S: 0.03 max; and Si: 0.70 max.

INCONEL® alloy C-276 (UNS N10276/W.Nr. 2.4819), anickel-molybdenum-chromium alloy with an addition of tungsten, isdesigned to have excellent corrosion resistance in a wide range ofsevere environments. High nickel and high molybdenum contents make thealloy especially resistant to pitting and crevice corrosion in reducingenvironments while chromium conveys resistance to oxidizing media. Thelow carbon content minimizes carbide precipitation during welding tomaintain corrosion resistance in as-welded structures. This alloy isresistant to the formation of grain boundary precipitates in the weldheat-affected zone, thus making it suitable for most chemical processapplications in an as-welded condition. INCONEL® alloy C-276 (UNSN10276/W.Nr. 2.4819) is widely used in the most severe environments,such as chemical processing, pollution control, pulp and paperproduction, industrial and municipal waste treatment, and recovery ofsour natural gas. The chemical composition of INCONEL® alloy C-276 (UNSN10276/W.Nr. 2.4819) is, by weight percent: Mo: 15.0-17.0; Cr:14.5-16.5; Fe: 4.0-7.0; W: 3.0-4.5; Co: 2.5 max; Mn: 1.0 max; C: 0.01max; V: 0.35 max; P: 0.04 max; S: 0.03 max; Si: 0.08 max; and remaindernickel.

INCONEL® alloy G-3 (UNS N06985/W. Nr. 2.4619), a solid solutionstrengthened nickel-chromium-iron-molybdenum alloy, provides anexcellent combination of mechanical properties and corrosion resistance,and has been used extensively in tubular goods used in hot, sourenvironments of the gas and oil industries. The compositional limits forINCONEL® alloy G-3 (UNS N06985/W. Nr. 2.4619) are generally indicated as21.0-23.5 wt % chromium, 18.0-21.0 wt % iron, 6.0-8.0 wt % molybdenum,1.5-2.5 wt % copper, a maximum of 0.50 wt % niobium plus tantalum, amaximum of 0.015 wt % carbon, a maximum of 1.5 wt % tungsten, a maximumof 1.0 wt % silicon, a maximum of 1.0 wt % manganese, a maximum of 0.04wt % phosphorus, a maximum of 0.03 wt % sulfur, a maximum of 5.0 wt %cobalt, and balance nickel. INCONEL® alloy G-3 (UNS N06985/W. Nr.2.4619) has excellent corrosion resistance to oxidizing chemicals andatmospheres and is resistant to reducing chemicals because of its nickeland copper contents. It also has exceptional stress-corrosion-crackingresistance in chloride-containing environments, very good resistance topitting and crevice corrosion, and good weldability and resistance tointergranular corrosion in the as-welded condition.

However, the current worldwide demand for ever increasing quantities ofgas and oil has caused the gas and oil industries to begin extractingthese commodities from deeper and sourer wells. Deeper wells mean highertemperatures, higher pressures, and more corrosive environments,especially with regard to sour gas environments. INCOLOY® alloy 028 (UNSN08028/W. Nr. 1.4563), INCONEL® alloy C-276 (UNS N10276/W.Nr. 2.4819),and INCONEL® alloy G-3 (UNS N06985/W. Nr. 2.4619) do not achieve a levelof strength and ductility desired for high strength applications for theoil and gas industries. Thus, there is a need for improvement in thecombination of mechanical properties and corrosion resistance comparedto what is currently offered by existing alloys. More specifically,there is a need to dramatically increase the strength of solid solutionnickel alloys without loss of ductility, toughness, fabricability, andcorrosion resistance. Furthermore, an alloy certified for oil and gasfield applications possesses a clean microstructure in addition to theusual required properties needed for any given application.

U.S. Pat. No. 4,400,210 to Kudo et al. (hereinafter “Kudo '210”)discloses an alloy described to be useful for manufacturing highstrength deep well casing, tubing, and drill pipes for use in oil welloperations. Kudo '210 states that the alloy exhibits improved resistanceto stress-corrosion-cracking in an H₂S—CO₂Cl⁻ environment. The alloycomprises: C: not more than 0.10%, Si: not more than 1.0%, Mn: not morethan 2.0%, P: not more than 0.030%, S: not more than 0.005%, N: 0-0.30%,Ni: 25-60%, Cr: 22.5-35%, Mo: less than 7.5% and W: less than 15%, andbalance iron with incidental impurities with the following equationsbeing satisfied: Cr(%)+10Mo(%)+5W(%)≧70%, and 3.5%≦Mo(%)+1/2W(%)<7.5%.Kudo '210 further states that the alloy may further comprise anycombination of the following: (i) one of Cu: not more than 2.0%, and/orCo: not more than 2.0%; (ii) one or more of rare earths: not more than0.10%; Y: not more than 0.20%, Mg: not more than 0.10%; and Ca: not morethan 0.10%; (iii) one or more of Nb, Ti, Ta, Zr, and V in the totalamount of from 0.5-4.0%; and (iv) nitrogen in an amount of 0.05-0.30%,preferably 0.10-0.25% may be intentionally added to the alloy. Kudo '210also states that nitrogen may be added in an amount of 0.05-0.25% incombination with Nb and/or V added in the total amount of 0.5-4.0%.However, Kudo '210 does not provide an alloy having a high strengthwithout loss of ductility, toughness, fabrication, and corrosionresistance while possessing a clean microstructure as required by theoil and gas industries.

U.S. Pat. No. 4,400,211 to Kudo et al. (hereinafter “Kudo '211”)discloses another alloy described to be useful for manufacturing highstrength deep well casing, tubing, and drill pipes for use in oil welloperations. Kudo '211 states that the alloy exhibits improved resistanceto stress-corrosion-cracking in an H₂S—CO₂—Cl⁻ environment. The alloycomprises: C: not more than 0.10%, Si: not more than 1.0%, Mn: not morethan 2.0%, P: not more than 0.030%, preferably not more than 0.003%, S:not more than 0.005%, Ni: 30-60%, Cr: 15-35%, at least one of Mo: notmore than 12%, and W: not more than 24%, and the balance iron withincidental impurities with the following equations being satisfied:Cr(%)+10Mo(%)+5W(%)≧110%, and 7.5%≦Mo(%)+1/2W(%)12%. Kudo '211 furtherstates that the alloy may further comprise any combination of thefollowing: (i) one of Cu: not more than 2.0%, and/or Co: not more than2.0%; (ii) one or more of rare earths: not more than 0.10%; Y, not morethan 0.20%; Mg: not more than 0.10%; and Ca: not more than 0.10%; (iii)one or more of Nb, Ti, Ta, Zr, and V in the total amount of from0.5-4.0%; and (iv) nitrogen in an amount of 0.05-0.30%, preferably0.10-0.25% may be intentionally added to the alloy. However, Kudo '211does not provide an alloy having a high strength without loss ofductility, toughness, fabrication, and corrosion resistance whilepossessing a clean microstructure as required by the oil and gasindustries.

SUMMARY OF THE INVENTION

According to one or more embodiments, it is an object to overcome one ormore problems of the related art.

It is another object to provide an alloy with increased strengthrelative to existing solid solution alloys, especially INCONEL® alloyG-3 (UNS N06985/W. Nr. 2.4619), while avoiding substantial losses ofductility, toughness, fabricability, and corrosion resistance andpossessing a clean microstructure as described by International StandardANSI/NACE MR0175/ISO15156-3 [Petroleum and Natural GasIndustries—Materials for use in H₂S-containing environments in oil andgas production].

These and other features and characteristics will become more apparentupon consideration of the following description and the appended claimswith reference to the accompanying figures, all of which form a part ofthis specification. As used in the specification and the claims, thesingular form of “a”, “an”, and “the” include plural referents unlessthe context clearly dictates otherwise.

Every effort has been made to correctly convert the units of measurementto SI units. To the extent that any discrepancy exists in theconversion, the non-SI units should be understood to be correct.

All compositions are described in weight percent unless expresslyspecified otherwise.

All yield strengths cited herein are measured at >0.2% offset unlessexpressly specified otherwise.

According to an exemplary embodiment of the present invention, asolid-solution nickel-based alloy for use in sour gas and oilenvironments, comprises, in percent by weight: chromium: min. of 21.0and max. of 24.0%; iron: min. of 17.0 and max. of 21.0%; molybdenum:min. of 6.5 and max. of 8.0%; copper: min. of 1.0 and max. of 2.5%;tungsten: min. of 0.1 and max. of 1.5%; sol. nitrogen: min. of 0.08 andmax. of 0.20%; manganese: max. of 4.0%; silicon: max. of 1.0%; carbon:max of. 0.015%; aluminum max of. 0.5%; and a total amount of niobium,titanium, vanadium, tantalum, and zirconium: max of 0.45%; the balancebeing nickel and incidental impurities.

The minimum chromium content is preferably 22.0%, more preferably 22.5%.The maximum chromium content is preferably 23.5%, more preferably 23.3%,and more preferably 23.1%.

The minimum iron content is preferably 18.0%, more preferably 19.0%. Themaximum iron content is preferably 20.5%, more preferably 20.0%.

The minimum molybdenum content is preferably 6.8%, more preferably 7.1%.The maximum molybdenum content is preferably 7.8%, more preferably 7.6%.

The minimum copper content is preferably 1.5%, more preferably 1.7%, andmore preferably 1.9%. The maximum copper content is preferably 2.3%,more preferably 2.1%.

The minimum tungsten content is preferably 0.50%, more preferably 0.90%.The maximum tungsten content is preferably 1.4%, more preferably 1.3%.

The minimum sol. nitrogen content is preferably 0.10%, more preferably0.12%. The maximum sol. nitrogen content is preferably 0.19%, morepreferably 0.18%, more preferably 0.17%, and more preferably 0.16%.

The maximum manganese content is preferably 2.0%, more preferably 1.0%.

The maximum silicon content is preferably 0.50%, more preferably 0.25%.

The maximum carbon content is preferably 0.010%, more preferably 0.005%.

The minimum aluminum content is preferably 0.001%, more preferably0.010%, and more preferably 0.10%. The maximum aluminum content ispreferably 0.40%, more preferably 0.30%.

The maximum total amount of niobium, titanium, vanadium, tantalum, andzirconium is preferably 0.40%. The maximum niobium content is preferably0.20%, more preferably 0.10%. The maximum tantalum content is preferably0.10%, more preferably 0.01%. The maximum vanadium content is preferably0.15%. The maximum tantalum content is preferably 0.10%. The maximumzirconium content is preferably 0.05%.

The maximum content of phosphorus is preferably 0.050%, more preferably0.010%, and more preferably 0.005%.

The maximum content of sulfur is preferably 0.050%, more preferably0.010%, and more preferably 0.005%.

The alloy may further comprise cobalt: maximum of 5.0%, preferablymaximum of 2.0%, more preferably maximum of 1.0%.

The alloy may further comprise one or more rare earth elements: maximumof 0.10%.

The alloy may further comprise yttrium: maximum of 0.20%.

The alloy may further comprise magnesium: maximum of 0.10%.

The alloy may further comprise calcium: maximum of 0.10%.

Boron, tin, lead, and zinc are each preferably controlled to be amaximum of 0.10% or less, preferably 0.01% or less, more preferably0.001% or less.

The minimum nickel content is preferably 40.0%, more preferably 42.0%,and more preferably 44.0%.

The content of Mo+½ W is preferably controlled to be a minimum of 7.6%,more preferably a minimum of 7.7%, and more preferably a minimum of7.8%. The content of Mo+½ W is preferably controlled to be a maximum of8.5%, more preferably a maximum of 8.4%, and more preferably a maximumof 8.3%.

The soluble nitrogen content is preferably a minimum of 0.08%, morepreferably a minimum of 0.10%, more preferably a minimum of 0.12%. Thesoluble nitrogen content is preferably a maximum of 0.20%, morepreferably a maximum of 0.18%, more preferably a maximum of 0.16%.

The alloy preferably has a low temperature ageability property,Δ(YS×El), of greater than 0 when a yield strength after aging (YS_(aa))is at least 145 ksi, wherein Δ(YS×El) is the 0.2% Offset Yield Strength(ksi) times Percent Elongation after aging (YS_(aa)×El_(aa)) minus the0.2% Offset Yield Strength (ksi) times Percent Elongation before aging(YS_(ba)×El_(ba)). In SI units, the alloy preferably has a lowtemperature ageability property, Δ(YS×El), of greater than 0 when ayield strength after aging (YS_(aa)) is at least 1000 MPa, whereinΔ(YS×El) is the 0.2% Offset Yield Strength (MPa) times PercentElongation after aging (YS_(aa)×El_(aa)) minus the 0.2% Offset YieldStrength (MPa) times Percent Elongation before aging (YS_(ba)×El_(ba)).

The alloy preferably has a low temperature ageability property thatsatisfies the condition, Δ(YS×El)≧600−(YS_(aa)−145)×20, wherein Δ(YS×El)is the 0.2% Offset Yield Strength (ksi) times Percent Elongation afteraging (YS_(aa)×El_(aa)) minus the 0.2% Offset Yield Strength (ksi) timesPercent Elongation before aging (YS_(ba)×El_(ba)). In SI units, thealloy preferably has a low temperature ageability property thatsatisfies the condition, Δ(YS×El)≧4138−(YS_(aa)1000)×20, whereinΔ(YS×El) is the 0.2% Offset Yield Strength (MPa) times PercentElongation after aging (YS_(aa)×El_(aa)) minus the 0.2% Offset YieldStrength (MPa) times Percent Elongation before aging (YS_(ba)×El_(ba)).

In an exemplary embodiment of the present invention, a method ofmanufacturing an article, includes: providing a billet formed from thealloy that is solution annealed and then cold worked a minimum of 20% toan article of predetermined dimensions; and heat treating the coldworked article at 468°-537° C. (875°-999° F.) for five minutes to eighthours.

In an exemplary embodiment of the present invention, an article ofmanufacture formed from the alloy has a cold-worked and agedmicrostructure, having grain boundaries with no continuous precipitates,wherein upon microstructural examination on a section taken in thelongitudinal direction with respect to the direction of cold work, atotal area fraction of intermetallic phases, nitrides, and carbides doesnot exceed 1.0%, and wherein an area fraction of sigma phase does notexceed 0.5%.

In an exemplary embodiment, the article has a 0.2% offset yield strengthof at least 1000 MPa (145 ksi) and at least 12% elongation.

In another exemplary embodiment, the article has a 0.2% offset yieldstrength of at least 1069 MPa (155 ksi) and at least 10% elongation.

In yet another exemplary embodiment, the article has a 0.2% offset yieldstrength of at least 1138 MPa (165 ksi) and at least 8% elongation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scatter chart showing an improved low temperature ageabilityproperty, which is evidenced by as an increase of a product of yieldstrength (YS) times elongation (El) as a result of a post cold work lowtemperature heat treatment (x-axis) vs. yield strength after heattreatment (y-axis).

FIG. 2 is a line chart showing the effect of cold work on yield strength(ksi) for a comparative alloy and an exemplary alloy, before and aftersalt bath heat treatment.

FIG. 3 is a line chart showing the effect of cold work on ultimatetensile strength (ksi) for the comparative alloy and the exemplaryalloy, before and after salt bath heat treatment.

FIG. 4 is a line chart showing the effect of cold work on percentelongation as a measurement of ductility for the comparative alloy andthe exemplary alloy, before and after salt bath heat treatment.

FIG. 5 shows a clean microstructure of an exemplary alloy having anitrogen content of approximately 0.14% and formed as a hot rolled platefollowed by heat treating at 1066° C. (1950° F.) for 30 minutes andwater cooling.

FIG. 6 shows a clean microstructure of an exemplary alloy having anitrogen content of approximately 0.14% and formed as a hot rolled platefollowed by heat treating at 1093° C. (2000° F.) for 30 minutes andwater cooling. The average grain size of the microstructure of FIG. 8was determined to be about 78 microns.

FIGS. 7 and 8 show the phases calculated to be present under equilibriumconditions for a comparative alloy.

FIGS. 9-11 show the phases calculated to be present under equilibriumconditions for an exemplary alloy.

FIG. 12 shows a Time-Temperature-Transformation (TTT) diagram for acomparative alloy.

FIG. 13 shows a Continuous-Cooling-Transformation (CCT) diagram for thecomparative alloy of FIG. 12.

FIG. 14 shows a Time-Temperature-Transformation (TTT) diagram for anexemplary alloy.

FIG. 15 shows a Continuous-Cooling-Transformation (CCT) diagram for theexemplary alloy of FIG. 14.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Every effort has been made to correctly convert the units of measurementto SI units. To the extent that any discrepancy exists in theconversion, the non-SI units should be understood to be correct.

This specification describes all compositions in weight percent unlessexpressly specified otherwise. All yield strengths cited herein aremeasured at a 0.2% offset, unless expressly specified otherwise.

Composition

Although the present invention is not limited by theory, the followingselected amounts of alloying elements in the solid-solution nickel-basedalloy tend towards the following beneficial effects.

Molybdenum (Mo): 6.5 to 8.0 Weight Percent

Molybdenum greatly contributes to strength development without degradingthermal stability during manufacture. The strengthening effect ofmolybdenum is primarily achieved by substitutional solid solutionstrengthening. Substitutional solid solution strengthening occurs bypositioning of molybdenum atoms at lattice positions within the matrixof the alloy. Due to a large difference in size between the smallmolybdenum atoms and the larger nickel, chromium, and iron atoms of thematrix, local stress fields are created around the molybdenum atoms thatinhibit the movement of dislocations through the lattice, thusincreasing the strength of the alloy. Also, molybdenum tends to improvecorrosion resistance, particularly resistance to pitting and crevicecorrosion, and molybdenum tends to contribute tostress-corrosion-cracking resistance and hydrogen embrittlementresistance. Accordingly, the molybdenum content is controlled to be atleast 6.5%, preferably at least 6.8%, and more preferably at least 7.1%.

However, excessive levels of molybdenum tend to form undesirable muphase [Mo₇(Ni, Fe, Cr)_(6]) and tend to contribute to formation ofundesirable sigma phase (FeMo)×(Ni, Co)₄, both of which negate againstsatisfying the clean microstructure requirement of the oil and gasindustries. Excessive molybdenum also tends to inhibit strengthdevelopment and impede corrosion resistance. Accordingly, the molybdenumcontent is controlled to be 8.0% or less, and preferably 7.8% or less,and more preferably 7.6% or less.

Sol. Nitrogen (N): 0.08 to 0.20 Weight Percent

Nitrogen is the primary element that distinguishes the alloy of thepresent invention from INCONEL® alloy G-3 (UNS N06985/W. Nr. 2.4619).Conventionally, nitrogen has been added to some types of nickel alloysto improve strength by precipitation of nitrides. However, precipitationof nitrides and other unwanted phases negates against the cleanmicrostructure requirement and tends to result in loss of ductility,toughness, and fabricability. For this reason, additions of high levelsof nitrogen have generally been avoided.

However, the present inventors have found that additions of high amountsof nitrogen that are less than the solubility limit surprisingly providea low-temperature ageability property that unexpectedly achieves animproved combination of strength and ductility after cold workingfollowed by a low temperature aging process. The improved combination ofstrength and ductility is achieved without precipitating excessiveamounts of unwanted phases and without significantly deterioratingtoughness, fabricability, or corrosion resistance.

Although the invention is not limited by theory, it is believed that thehigh amounts of nitrogen in solid solution increase the strength of thealloy by interstitial solid solution strengthening, and it is believedthat soluble nitrogen may, as a result of the low temperature heattreatment, interact with dislocations in the lattice of the alloy thathave resulted from cold working to provide an improved combination ofstrength and ductility. Accordingly, the sol. nitrogen content of thealloy is controlled to be at least 0.08%, preferably at least 0.10%, andmore preferably at least 0.12%.

However, levels of nitrogen exceeding the solubility limit of the alloytend to precipitate nitrides and/or carbonitrides. Excessive amounts ofthese precipitates tend to result in loss of ductility, toughness, andfabricability, negate against a clean microstructure, and also rob thematrix of the elements—molybdenum, tungsten, and chromium—therebyreducing the strength and the corrosion resistance of the alloy.Accordingly, the sol. nitrogen content of the alloy is 0.20% or less,preferably 0.19% or less, more preferably 0.18% or less, more preferably0.17% or less, and more preferably 0.16% or less.

At slightly beyond the solubility limit of nitrogen, it is possible thata degree of strengthening may result from Cr₂N dispersoids within thematrix grains. However, greater amounts of nitrogen tend to contributeto grain boundary film formation that negates against the cleanmicrostructure requirement and contributes to poor properties asdiscussed above. Accordingly, the overall nitrogen content of the alloyis preferably 0.20% or less, more preferably 0.19% or less, morepreferably 0.18% or less, more preferably 0.17% or less, and morepreferably 0.16% or less. The insoluble nitrogen content of the alloy ispreferably 0.12% or less, more preferably 0.10% or less, more preferably0.08% or less, more preferably 0.06% or less, more preferably 0.04% orless, and more preferably 0.02% or less.

Nitrogen may be added by dissolving nitrogen gas into the molten alloyprior to tapping. The dissolved nitrogen becomes part of the solidsolution during cooling and thereby strengthens the alloy by solidsolution strengthening. Since the nitrogen remains in solid solution,nitrogen is trapped in the matrix and does not precipitate. To maximizethe solid solution strengthening effect of nitrogen, the solubilitylimit of nitrogen in the alloy is preferably maximized. The solubilitylimit is affected by the presence of other alloying elements in thenickel alloy. Thus, the selection of alloying elements is preferablyoptimized to ensure that high levels of nitrogen remain in solidsolution. To achieve the intended effect, the maximum solubility ofnitrogen in the alloy at 1538° C. (2800° F.) is preferably at least0.12%, more preferably at least 0.14%, and most preferably at least0.16%.

The following Table 1 shows the effect of composition (in percent byweight) on the maximum solubility limit of nitrogen at 1538° C. (2800°F.), which is approximately 93° C. (200° F.) above the temperature oftapping, as well as the compositional effect on the sigma solvus whichis discussed later in the application.

TABLE 1 1A 1B 1C 1D 1E C 0.007 0.007 0.007 0.007 0.004 Ni 45.8 42.4 44.946.9 47.0 Co 0.6 0.6 0.6 0.6 0.0 Al 0.23 0.23 0.23 0.23 0.20 Ti 0.010.01 0.01 0.01 0.005 Nb 0.13 0.13 0.13 0.13 0.19 Mo 6.9 6.9 6.9 7.5 7.0Fe 19.4 19.5 17.0 14.5 19.5 Cr 22.9 22.9 22.9 22.9 22.2 W 0.94 0.94 0.940.94 0.92 Si 0.17 0.17 0.17 0.17 0.21 Cu 2.0 2.0 2.0 2.0 2.0 Mn 0.75 4.04.0 4.0 0.72 Max. N₂ 0.15 0.18 0.18 0.15 0.20 solubility at 2800° F.Sigma 1900 2074 1998 1994 1854 Solvus ° F.

As shown in the above Table 1, the composition of the alloy may beoptimized to achieve a high maximum nitrogen solubility.

Chromium (Cr): 21.0 to 24.0 Weight Percent

Chromium beneficially increases the solubility limit of nitrogen in thealloy, thereby increasing the amount of nitrogen that may be added.Also, chromium provides resistance to oxidizing environmentsparticularly during manufacture, and chromium provides pittingresistance to chloride-containing environments and contributes toresistance to stress corrosion cracking and hydrogen embrittlement.Accordingly, the chromium content is controlled to be at least 21.0%,preferably at least 22.0%, and most preferably at least 22.5%

However, chromium has a strong affinity for carbon. In the presence ofcarbon, chromium forms undesirable carbides and carbonitridesparticularly along the grain boundaries, which decreases the amount ofnitrogen in solid solution, deters from fabricability, and negatesagainst the requirement for a clean microstructure. Accordingly, thechromium content is controlled to be at most 24.0%, preferably at most23.5%, more preferably at most 23.3%, and more preferably at most 23.1%.

Carbon (C): 0.015 Weight Percent or Less

Small amounts of carbon generally remain in the alloy as a result ofnormal processing conditions. However, excessive amounts of carbon causethe formation of chromium carbides and carbonitrides, particularly alongthe grain boundaries, which deplete the amount of chromium in solutionnear the grain boundaries. This grain boundary depletion of chromiumcauses intergranular corrosion, particularly in weld heat-affectedzones. The precipitation of carbides and carbonitrides also prevents thealloy from meeting industry requirements for a clean microstructure.Excessive carbon also robs the matrix of the strengtheningelements—molybdenum, tungsten, and chromium—thereby reducing thestrength and the corrosion resistance of the alloy. Accordingly, thecarbon content is controlled to be 0.015% or less, preferably 0.010% orless, and more preferably 0.005% or less.

Tungsten (W): 0.10 to 1.5 Weight Percent

As explained above, small amounts of carbon generally remain in thealloy as a result of normal processing conditions. Tungsten has abeneficial effect of tying up carbon as intragranular WC and thusminimizing the detrimental formation of chromium carbides andcarbonitrides. Also, tungsten is believed to contribute to tensilestrength, weldability, pitting resistance, resistance to stresscorrosion cracking, and hydrogen embrittlement resistance. To achievethe desired effect, the tungsten content is controlled to be at least0.1%, preferably at least 0.50%, and more preferably at least 0.90%

However, tungsten (like molybdenum) detrimentally increases thepropensity of mu phase, which negates against the requirement for aclean microstructure and deteriorates strength development and corrosionresistance. Excessive tungsten also adds to the cost of the alloy whilesaturating the beneficial effects. Accordingly, the tungsten content iscontrolled to be 1.5% or less, preferably 1.4% or less, and morepreferably 1.3% or less.

Copper (Cu): 1.0 to 2.5 Weight Percent

Copper is beneficial for increasing the resistance to sour gascorrosion, which is particularly important in deep sour wells prone toH₂S environments. Accordingly, the copper content is controlled to be atleast 1.0%, preferably at least 1.5%, and more preferably at least 1.7%,and more preferably at least 1.9%.

However, excessive amounts of copper lower the resistance to pittingcorrosion, which is detrimental to the alloy if in the presence ofchlorides. Accordingly, the copper content is controlled to be 2.5% orless, preferably 2.3% or less, and more preferably 2.1% or less.

Combined Amount of Molybdenum (Mo) and Tungsten (W): Mo+½ W=7.6 to 8.5Weight Percent

As discussed above, both molybdenum and tungsten are effective forincreasing the strength of the alloy. To ensure a high level ofstrength, a combination of molybdenum and tungsten are preferablycontrolled such that a content of Mo+½ W is high. Accordingly, a contentof Mo+½ W is preferably controlled to be at least 7.6%, more preferablyat least 7.7%, and most preferably at least 7.8%.

However, an excess combined amount of molybdenum and tungsten tends todegrade the stability of the alloy by the precipitation of unwantedphases, which negate against the requirement for a clean microstructureand deteriorates strength development and corrosion resistance.Accordingly, the Mo+½ W content is preferably controlled to be 8.5% orless, more preferably 8.4% or less, and most preferably 8.3% or less.

Niobium (Nb), Titanium (Ti), Tantalum (Ta), Zirconium (Zr), and Vanadium(V): Total Combined Amount of 0.45 Weight Percent or Less

As explained above, nitrogen is added to the alloy to provide a lowtemperature ageability property. To ensure that nitrogen remains insolid solution during cooling, the addition of high-temperature nitrideforming elements are limited, the presence of which would precipitatedetrimental amounts of nitrides, thereby reducing the amount of solublenitrogen and degrading ductility, toughness, and fabricability.Accordingly, the total combined amount of niobium, titanium, tantalum,zirconium, and vanadium is controlled to be 0.45% or less, preferably0.40% or less. The niobium content is preferably controlled to be 0.20%or less. The titanium content is preferably controlled to be 0.10% orless, more preferably 0.01% or less. The vanadium content is preferablycontrolled to be 0.15% or less. The tantalum content is preferablycontrolled to be 0.10% or less. The zirconium content is preferablycontrolled to be 0.05% or less.

Iron (Fe): 17.0 to 21.0 Weight Percent

Chromium and molybdenum may be economically included in the alloy by theaddition of ferrochromium and ferromolybdenum to improve the commercialviability of the alloy. Accordingly, the iron content is preferablyincluded at an amount of at least 17.0%, preferably at least 18.0%, andmore preferably at least 19.0%.

However, the addition of excessive amounts of iron favors undesirablesigma phase formation, which negates against the requirement for a cleanmicrostructure. Accordingly, the iron content is controlled to be atmost 21.0%, preferably 20.5% or less, and more preferably 20.0% or less.

Phosphorus (P): 0.050 Weight Percent or Less

Phosphorus may be present in the alloy as an impurity. Excessive amountsof phosphorus may cause susceptibility to hydrogen embrittlement.Accordingly, the phosphorus content is preferably controlled to be0.050% or less, more preferably 0.010% or less, and most preferably0.005% or less.

Sulfur (S): 0.050 Weight Percent or Less

Sulfur may be present in the alloy as an impurity. Excessive amounts ofsulfur may deteriorate the hot workability of the alloy. Accordingly,the sulfur content is preferably controlled to be 0.050% or less, morepreferably 0.010% or less, and most preferably 0.005% or less.

Aluminum (Al): 0.5 Weight Percent or Less

Aluminum is beneficial for desulfurizing the alloy. To achieve thedesired effect, the aluminum content is preferably controlled to be atleast 0.001%, more preferably at least 0.010%, and more preferably atleast 0.10%.

However, excessive amounts of aluminum detrimentally contribute to theformation of undesirable intermetallic phases. Accordingly, the aluminumcontent is controlled to be at most 0.5%, preferably at most 0.40%, andmore preferably at most 0.30%.

Manganese (Mn): 4.0 Weight Percent or Less

Manganese is beneficial for desulfurizing the alloy, especially inabsence of high amounts of aluminum. However, excessive amounts ofmanganese reduce the resistance of the alloy to acid chlorides.Accordingly, the manganese content is controlled to be 4.0% or less,preferably 2.0% or less, and more preferably 1.0% or less.

Silicon (Si): 1.0 Weight Percent or Less

Silicon is beneficial for increasing the oxidation resistance of thealloy particularly during manufacture. However, excessive amounts ofsilicon reduce hot and cold workability of the alloy. Also, silicon is astrong nitride former. Thus, excessive amounts of silicon may reduce thebeneficial effect of nitrogen and negate against the cleanmicrostructure requirement. Accordingly, the silicon content iscontrolled to be 1.0% or less, preferably 0.50% or less, and morepreferably 0.25% or less.

Cobalt (Co): 5.0 Weight Percent or Less

Cobalt may be beneficial for further improving the corrosion resistanceof the alloy. Therefore, cobalt may be added when especially highcorrosion resistance is required. However, excessive amounts of cobaltgreatly add to the cost of the alloy. Accordingly, the cobalt content ispreferably controlled to be 5.0% or less, more preferably 2.0% or less,and most preferably 1.0% or less.

Rare Earths, Y, Mg, and Ca

Rare Earths Y, Mg, and Ca may all be beneficial for improving hotworkability. Therefore, when the alloy has to be subjected to severe hotworking, it may be desirable to incorporate at least one of theseelements in the alloy. However, excessive amounts of these elements maydeteriorate the properties of the alloy. Accordingly, the content ofrare earths is preferably controlled to be 0.10% or less, the content ofY is preferably controlled to be 0.20% or less, the content of Mg ispreferably controlled to be 0.10% or less, and the content of Ca ispreferably controlled to be 0.10% or less.

Nickel (Ni)

The main benefit of nickel is to maintain a stable austeniticsingle-phase structure and assure stability and cleanliness of themicrostructure, which is important for obtaining optimum corrosionresistance capable of being economically produced and fabricated.Accordingly, nickel may be included as the balance of the alloycomposition. The nickel content is preferably controlled to be 40.0% ormore, more preferably 42.0% or more, and more preferably 44.0% or more.

Incidental Elements

The alloy may include incidental elements, such as B, Sn, Pb, and Zn.However, excessive amounts of these elements may deteriorate theproperties of the alloy. Accordingly, each incidental element ispreferably controlled to be 0.10% or less, more preferably 0.01% orless, and more preferably 0.001% or less.

The above description describes the amounts and effects of the primaryelements in the alloy as best understood. While the invention isenvisioned to encompass any alloy having amounts of elements within theabove-described ranges, the addition of amounts of additional elementsthat would materially affect the basic and novel characteristics of thepresent invention should be avoided. In an embodiment of the presentinvention, the alloy consists essentially of or consists of amounts ofelements within the above-described ranges, along with incidentalimpurities, while ensuring increased strength and without detrimentalloss of ductility, toughness, fabricability, and corrosion resistance.

Method of Manufacture

In an exemplary embodiment of the present invention, there is a methodof manufacturing an article formed from a solid solutionnickel-chromium-iron-molybdenum corrosion resistant alloy as describedabove. The alloy is preferably processed in the manner described below.Although the present invention is not limited by theory, it is believedthat the following processing steps have the effects described below.

Introduction of Nitrogen Gas Into the Molten Alloy

Nitrogen may be added by bubbling nitrogen gas into the molten metalalloy prior to tapping. The introduced nitrogen becomes part of thesolid solution during solidification. If the amount of nitrogen is lessthan the solubility limit, nitrogen is trapped in the matrix and doesnot precipitate. It is believed that nitrogen in solid solution causesinterstitial solid solution strengthening of the alloy as previouslydescribed. The solubility limit of nitrogen in the alloy is affected bythe additions of other alloying elements, which are selected in thealloy of the present invention to ensure that high levels of nitrogenremain in solid solution. For example, chromium, in particular, isbelieved to substantially increase the solubility limit of nitrogen inthe alloy, thereby increasing the amount of nitrogen that may be addedwithout exceeding the solubility limit.

Homogenization Time and Temperature

Homogenizing has a beneficial effect of dissolving undesirable secondphases that may form during cooling as a result of segregation ofalloying elements. To achieve the beneficial effect, a temperature ofthe homogenization is preferably controlled to be at least 1093° C.(2000° F.), and a time of the homogenization is preferably controlled tobe at least 20 hours.

However, an excessively high temperature during homogenization may causeincipient melting of the alloy. Accordingly, a temperature of thehomogenization is preferably controlled to be 1232° C. (2250° F.) orlower.

Also, an excessively high time of homogenization is believed to saturatethe effect of the homogenization treatment while raising the processingcost. Accordingly, a time of the homogenization is preferably controlledto be 48 hours or less.

Hot Working Temperature

Hot working has a beneficial effect of changing the shape of a castingot into the general desired shape of the billet and doing so at hightemperatures at which the alloy is more easily deformed. Hot working mayinclude extrusion for tube making, forging for bars and flats, and hotrolling for sheets and plates. To achieve the beneficial effect of hotworking, a temperature of the hot working is preferably controlled to be1149° C. (2100° F.) or greater. Excessively low temperatures of hotworking may cause an increase of flow stress that exceeds the capacityof hot working equipment.

However, an excessively high temperature of hot working may causeadiabatic heating that results in incipient melting of the alloy.Accordingly, a temperature of the hot working is preferably controlledto be 1204° C. (2200° F.) or less.

Solution Annealing or Hot Roll Solution Annealing

Solution annealing is performed to ensure that the constituents of thealloy are in solid solution. Hot roll solution annealing is performedwhen annealing and substantial reduction of the material is to beaccomplished during annealing, and for flow stress reasons one does notwant the material to get cold because of the increase in flow stress.The temperature at which the alloys are annealed is preferably chosen tobe well above the recrystallization temperature and at a temperatureadequate to keep the flow stress low. Experience indicates that solutionannealing at 1038° C. (1900° F.) for a minimum of 20 minutes issufficient to assure the completion of the anneal. The need for a hotroll solution anneal is a function of a number of variables, such ashandling time to reach the hot roll press, time on the press, and size.

Cold Working

Cold working has a beneficial effect of strengthening the alloy by theintroduction of defects into the lattice of the alloy. Cold workingcreates a textured microstructure that is not necessarily homogeneouswith regard to mechanical properties in the longitudinal and transversedirections. Cold working may include drawing, pilgering, swaging, rollforming, and cold rolling of flats.

To achieve the desired level of strengthening, the amount of coldworking is preferably controlled to be at least 20%, more preferably atleast 25%, and most preferably at least 30%. When the cold working isdrawing, the amount of cold working is preferably controlled to be atleast 25%, more preferably at least 30%, and most preferably at least35%.

However, the ability to cold work a material depends on the capabilityof the equipment to work the alloy or the capacity of the material toaccept the cold working. Accordingly, an amount of cold working isgenerally less than 80%, preferably less than 70%, and more preferablyless than 60%.

Low Temperature Heat Treatment

For precipitation hardenable nickel alloys, it is common to perform aheat treatment at temperatures ranging from 427° C. (800° F.) to 871° C.(1600° F.) to increase the strength of the alloy by precipitation of adispersed phase throughout the matrix. This precipitation strengtheningeffect is caused by the precipitation of submicroscopic particlesthroughout the matrix, which results in a marked increase in hardnessand strength. Principal aging phases in precipitation-hardenable highnickel alloys usually include one or more of gamma prime (Ni₃Al orNi₃Al, Ti), eta (Ni₃Ti), and gamma double prime (bct-Ni₃Nb). Otherphases that may be present include carbides (such as M₂₃C₆, M₇C₃, M₆C,and MC), nitrides (MN) and carbonitrides (MCN), and borides (M₃B₂), aswell as Laves phase (M₂Ti) and delta phase (orthorhombic-Ni₃Nb).

However, according to the exemplary method of the present invention, thelow-temperature heat treatment may be applied to the alloy, which isdeemed to be a non-precipitation hardenable alloy. Instead, the lowtemperature heat treatment is applied to a solid solution strengthenedand cold worked alloy. The inventors of the present application havefound that the low temperature heat treatment applied to the alloyhaving high levels of nitrogen in amounts that are less than thesolubility limit surprisingly provides a low temperature ageabilityproperty that unexpectedly achieves an improved combination of strengthand ductility without precipitating unwanted phases and withoutdeteriorating toughness, fabricability, and corrosion resistance.

Although the invention is not limited by theory, it is believed that thetemperature of the heat treatment, although less than the temperature atwhich effects of cold working are completely removed, may partiallyremove the effect of the cold work since the temperature is in thestress relieving temperature range. It is also believed that, as aresult of the low temperature heat treatment, nitrogen in solid solutioninteracts with the dislocations in the lattice of the alloy that wereintroduced by the cold working step. The exact mechanism explaining whythe low temperature heat treatment enhances the mechanical properties isnot clear. Although the present invention is not limited by theory, itis also believed that cooling after low temperature aging treatment maycreate compressive stresses on the alloy resulting in the strengthening.

In a preferred embodiment, the low temperature heat treatment may beachieved by employing a hot salt bath. When an article is immersed inmolten salt, heat is transferred by direct contact from the molten saltto the surface of the article. Also, since articles are immersed in thesalt bath, air cannot contact the article and, therefore, scaling,oxidation, and decarburization are thereby avoided. Also, heat transferinto materials by salt bath treatment is very rapid—faster than withradiation methods, thereby reducing the amount of time necessary for theheat treatment.

Time and Temperature of Heat Treatment

To achieve the desired effect of the low temperature heat treatment, atemperature of the heat treatment is preferably controlled to be 468° C.(875° F.) or greater, more preferably 482° C. (900° F.) or greater, mostpreferably 496° C. (925° F.) or greater. A time of the heat treatment iscontrolled to be 5 minutes or greater, preferably controlled to be 15minutes or greater, more preferably 30 minutes or greater, and mostpreferably 45 minutes or greater.

However, excessively high temperatures are believed to decrease thestrength enhancement of the heat treatment. Also, the salt bath is notstable at excessively high temperatures. Accordingly, a temperature ofthe heat treatment is preferably controlled to be 537° C. (999° F.) orless, more preferably 524° C. (975° F.) or less, and more preferably510° C. (950° F.) or less.

Additionally, for an excessively long time of heat treatment, the effectis believe to be saturated. Accordingly, a time of heat treatment ispreferably controlled to be 8 hours or less, more preferably 4 hours orless, and more preferably 2 hours or less.

Properties Low Temperature Ageability

As explained above, low temperature heat treatments are typicallyapplied to precipitation hardenable alloys rather than solid solutionstrengthened alloys. However, it was surprisingly found that the lowtemperature heat treatment applied to the solid solution strengthenedalloy after cold working unexpectedly achieved an improved combinationof strength and ductility. More specifically, the alloys surprisinglyexhibited an improved combination of 0.2% offset yield strength (YS) andelongation as a result of the low temperature heat treatment to achievea high combined amount of yield strength and ductility. This effect canbe further understood with reference to the following experimentalexamples.

The alloy compositions of Tables 2 and 3 were air cast as 22.7 Kg (50lbs) heats and alloys 2A, 2B, and 2C were homogenized at 1204° C. (2200°F.) for 36 hours and air cooled, followed by hot rolling at 1177° C.(2150° F.) with a 10:1 ratio to simulate extrusion, followed by a waterquench and post hot rolling anneal at 1010° C. (1850° F.) for 1 hourfollowed by water quench for subsequent cold reduction trials asspecified in Table 4. Alloys 3A and 3B were similarly homogenized at1204° C. (2200° F.) for 24 hours and air cooled. Alloys 3A and 3B werethen hot rolled at 1190° C. (2175° F.) to 11.4 mm (0.45 inch) plate andair cooled. A post hot rolled solution anneal was conducted at 1066° C.(1951° F.) for 30 minutes and subsequently water quenched. The alloys ofTables 2 and 3 were then cold reduced by rolling at specified amountslisted in Tables 4 and 5 and a portion of the rolled plate was heattreated at 500° C. (932° F.) in a salt bath for 45 minutes followed byair cooling.

TABLE 2 Three Comparative Compositions Heat C Mn Fe Si Cu Ni Cr Al Ti MoW N 2A 0.003 0.71 19.1 0.20 1.96 49.7 22.0 0.15 0.004 6.02 0.02 0.020 2B0.004 0.72 19.5 0.21 2.02 47.0 22.2 0.19 0.005 6.99 0.92 0.020 2C 0.0020.70 19.6 0.18 1.97 46.0 22.6 0.22 0.000 7.03 1.01 0.023

TABLE 3 Two Exemplary Compositions Heat C Mn Fe Si Cu Ni Cr Al Ti Mo W N3A 0.004 0.7 18.8 0.16 1.99 45.7 22.9 0.16 <0.001 7.52 1.26 0.134 3B0.003 0.73 19.5 0.19 2.01 45.5 22.9 0.19 0.0013 7.19 1.08 0.15

TABLE 4 Lists the key tensile properties and hardness of the comparativecompositions 0.2% Yield 0.2% Yield Strength after % Elongation % ColdStrength % Heat Treatment after Heat Heat Work MPa/ksi Elongation HRCMPa/ksi Treatment HRC 2A 20 807/117.0 20.5 29 810/117.5 21 29 401069/155.0  8.7 33 1165/169.0  7.1 36 2B 20 876/127.0 15.8 31 903/131.016.2 33 40 1124/163.0  8.5 36 1248/181.0  5.2 41 2C 30 875/126.9 17.2 31952/138.0 17.8 34

TABLE 5 Lists the key tensile properties and hardness of the compositionwithin the scope of this invention 0.2% Yield Strength after 0.2% YieldHeat % Cold Strength % Treatment % Elongation after Heat Work MPa/ksiElong. HRC MPa/ksi Heat Treatment HRC 3A 30  999/144.9 18.1 361039/150.7 21.5 37 35 1034/149.9 14.8 34 1085/157.4 17.4 37 401071/155.4 13.5 34 1145/166.1 17.1 39 Charpy 0.2% Yield Impact CharpyStrength after Heat % 0.2% Yield Impact after Heat % Elong. TreatmentHRC after Cold Strength Ft. Treatment after Heat Ft. Heat Heat WorkMPa/ksi % Elong. Lbs.* HRC MPa/ksi Treatment Lbs./J/cm²* Treatment 3B 301076/156 15.5 32 33 158 18.4  32/108.5 32 35 1089/158 11.0 29 35 15815.5 28/94.9 32 40 1096/159 8.0 25 37 172 8.0 23/78.0 35 *Charpy impactvalves are reported for half-size samples at a test temperature of 14°F. For conversion into SI unites, the value reported in Ft. Lbs. ismultiplied by 3.39 to find the value on Joules/cm² at −10° C.

FIG. 1 shows an improved low temperature ageability property of theexemplary alloys 3A and 3B from Tables 3 and 5 relative to comparativealloys 2A, 2B, and 2C from Tables 2 and 4. More specifically, FIG. 1plots a change in the product of 0.2% offset yield strength (ksi) andelongation (El) as a result of aging, i.e., Δ(YS×El), vs. the yieldstrength (ksi) after aging (YS_(aa)), wherein Δ(YS×El) is the 0.2%Offset Yield Strength (ksi) times Percent Elongation after aging(YS_(aa)×El_(aa)) minus the 0.2% Offset Yield Strength (ksi) timesPercent Elongation before aging (YS_(ba)×El_(ba)).

As can be understood from FIG. 1, the high sol. nitrogen exemplaryalloys 3A and 3B of the present invention have an excellent improvementin the combination of yield strength and elongation as a result of thelow temperature aging process when the alloys are cold worked asufficient amount to have a high yield strength of at least 145 ksi inthe heat treated condition, relative to the low nitrogen-containingcomparable alloys 2A, 2B, and 2C.

As shown by FIG. 1, the change in product of yield strength (ksi) andelongation as a result of aging Δ(YS×El) is greater than 0 when a finalyield strength after aging (YS_(aa)) of the alloy is at least 145 ksi.Thus, the alloys of the present invention show the ability to improvethe combined yield strength and elongation of high yield strength alloysby applying a low temperature heat treatment after cold working.

Furthermore, as represented in FIG. 1, the above-described lowtemperature ageability property of the alloys can be alternativelyexpressed by the following formula: Δ(YS×El)≧600−(YS_(aa)−145)×20,wherein Δ(YS×El) is the 0.2% Offset Yield Strength (ksi) times PercentElongation after aging (YS_(aa)×El_(aa)) minus the 0.2% Offset YieldStrength (ksi) times Percent Elongation before aging (YS_(ba)×El_(ba)).This formula expresses a distinction between the exemplary alloys andthe comparative alloys—the exemplary alloys have a better combined yieldstrength and elongation for a given final yield strength.

In contrast to the exemplary alloys, the comparative alloys, when coldworked enough to have a high final yield strength of at least 145 ksi,have a decreased product of yield strength and elongation as a result ofaging, and the comparative alloys only achieve the improved combinationof yield strength and elongation when cold worked a low amount such thata low final yield strength of less than 145 ksi is achieved.

Although the present invention is not limited by theory, it is believedthat, after heat treatment, the soluble nitrogen may interact withdislocations that result from cold working to provide an improvedcombination of strength and ductility. Thus, a high amount of coldworking is believed to contribute to the beneficial interaction betweenthe nitrogen in solid solution and the cold worked structure.

Mechanical Properties—Strength, Ductility, and Toughness

In an exemplary embodiment, the alloys, after cold working andapplication of the low temperature heat treatment, achieve a minimum0.2% offset yield strength of at least 1000 MPa (145 ksi), preferably1069 MPa (155 ksi), and more preferably 1138 MPa (165 ksi). The alloysalso achieve a minimum elongation of 8.0%, preferably at least 10.0%,more preferably at least 12.0%, more preferably at least 14.0%, morepreferably at least 16.0%, and more preferably 19.0%.

The alloys also target combinations of mechanical properties including:

-   -   (i) a 0.2% offset yield strength of at least 1000 MPa (145 ksi)        with at least 12% elongation, preferably at least 15%, and more        preferably at least 19%;    -   (ii) a 0.2% offset yield strength of at least 1069 MPa (155 ksi)        with at least 10% elongation, preferably at least 13%, and more        preferably at least 16%; and    -   (iii) a 0.2% offset yield strength of at least 1138 MPa (165        ksi) with at least 8.0% elongation, preferably at least 10%, and        more preferably at least 12%.

For comparative alloys 2A, 2B, and 2C, it is observed as shown in Table4 that when the % cold work achieves the target 0.2% yield strength of1000 MPa (145 ksi), the ductility as measured by the % elongation isbelow the acceptable minimum of 12%, and when the % cold work achievesthe target 0.2% yield strength of 1069 MPa (155 ksi), the ductility asmeasured by the % elongation is below the acceptable minimum of 10%, andwhen the % cold work achieves the target 0.2% yield strength of 1138 MPa(165 ksi), the ductility as measured by the % elongation is below theacceptable minimum of 8%.

Thus, comparative alloys 2A, 2B, and 2C do not achieve any of the targetcombinations of properties listed in the paragraph above. The deficientmechanical properties of comparative alloys 2A, 2B, and 2C areattributed to the lack of nitrogen for solid solution strengthening inthe composition. As seen in Table 4, the use of a low temperature heattreatment for the low nitrogen alloys 2A, 2B, and 2C does not resolvethe problem and in fact exacerbates the problem by decreasing thecombination of yield strength and elongation when the comparative alloysare cold worked to yield strength exceeding 1000 MPa (145 ksi).

This problem with the comparative alloys is resolved by the addition ofnitrogen as shown in Table 5, thereby achieving the targetedcombinations of mechanical properties outlined above. More specifically,a 0.2% offset yield strength of at least 1000 MPa (145 ksi) with anelongation of at least 12% is achieved, preferably at least 15%, morepreferably at least 19%. Also, a 0.2% offset yield strength of at least1069 MPa (155 ksi) with an elongation of at least 10% is achieved,preferably at least 13%, more preferably at least 16%. Additionally, a0.2% offset yield strength of at least 1138 MPa (165 ksi) with anelongation of at least 8% is achieved, preferably at least 10%, morepreferably at least 12%.

In addition to meeting the targeted combination of final mechanicalproperties, the exemplary alloys also preferably meet the propertyrequirements of ANSI/NACE MR0175/ISO15156-3 for Type 4e alloys. For coldwork reductions of 40%, the property requirements of ANSI/NACEMR0175/ISO15156-3 for Type 4e alloys requires that a 0.2% offset yieldstrength does not exceed 1240 MPa (180 ksi) and that hardness does notexceed HRC 45. Thus, the alloys advantageously meet the mechanicalproperties of Type 4e while advantageously having the morecost-effective composition of Type 4d alloys. Type 4d alloys arerequired to have a minimum 19.0wt. % Cr, a minimum Ni+Cr of 45wt %, anda minimum Mo+W of 6wt %, whereas Type 4e alloys are required to have aminimum 14.5wt % Cr, a minimum Ni+Cr of 52wt %, and a minimum Mo of 12wt%. Thus, Type 4e alloys require greater amounts of Ni+Cr and greater Mocontents, thereby substantially increasing the cost of Type 4e alloysrelative to Type 4d alloys. The alloys of the present inventionpreferably advantageously have the composition of a Type 4d alloy whilemeeting the mechanical property requirements of a Type 4e alloy. Thisresults in a substantial cost advantage because oil and gas companiescan use the improved alloy where high strength and good corrosionresistance are required. This is particularly beneficial for HPHT (highpressure high temperature) wells which are currently commonly developedsince shallow wells are generally depleted.

The improved strength of the alloys as a result of the low temperatureheat treatment is additionally important because ANSI/NACE MR0175/ISO15156-3 requires that Type 4d alloys have a maximum yield strength of150 ksi (1034 MPa) and maximum 40 HRC in the annealed and cold-workedcondition when the alloy is given cold-work reductions of 30 and 35%.Additionally, ANSI/NACE MR0175/ISO 15156-3 requires that Type 4e alloyshave a maximum yield strength of 180 ksi (1240 MPa) and a maximum 45 HRCin the annealed and cold-worked condition when the alloy is given acold-work reduction of 40%. Accordingly, the alloys are able to remainbelow the required maximum yield strengths and hardness values in theannealed and cold worked condition, and then subsequently increase theyield strength beyond the maximum amounts as a result of the lowtemperature heat treatment.

The following additional experimental examples of Table 6 and FIGS. 2-4further illustrate the advantageous mechanical properties of the alloys.The following examples relate to mechanical properties of cold workedvs. cold worked plus heat treated samples for comparative alloy 6Ahaving a low nitrogen content of less than 0.03% vs. an exemplary alloy6B having a high nitrogen content (0.15%).

TABLE 6 Chemical composition of 100 lbs Alloy 6A and 6B ALLOY 6A 6B C0.004 0.003 Mn 0.73 0.73 Fe 19.5 19.5 S 0.0016 0.0017 Si 0.17 0.19 Cu 22 Ni 45.6 45.5 Cr 22.8 22.9 Al 0.22 0.19 Ti 0 0.0013 Mg 0.006 0.011 Co0.46 0.46 Mo 7.1 7.2 Nb 0.15 0.15 Ta 0.003 0.003 P 0.0041 0.0044 Ca0.0001 0.0001 N 0.027 0.15 V 0.012 0.014 W 1.07 1.08

FIGS. 2-4 illustrate a comparison of mechanical properties of flatcold-rolled comparative alloy 6A and exemplary alloy 6B with and withoutsalt bath treatment. As can be understood from FIGS. 2-4, exemplaryalloy 6B results in significantly higher yield strength and highertensile strength, without a significant decrease in elongation comparedto comparative alloy 6A.

Microstructure

The microstructure of the exemplary alloys has an austenitic matrix anda cleanliness that satisfies the requirements of the oil and gasindustries. The clean microstructure is believed to be especiallyimportant for ensuring the manufacture of a cracking-resistant material.

The clean microstructure requirement is determined after an articleformed from the alloy is cold worked and subjected to a low temperatureheat treatment as described previously above. A clean microstructure isdeemed to be a microstructure having, upon microstructural examinationon a section taken in the longitudinal direction with respect to thedirection of cold work, grain boundaries with no continuousprecipitates, with intermetallic phases, nitrides and carbides notexceeding 1.0% in total area, and with sigma phase not exceeding 0.5% intotal area. Failure to achieve a clean microstructure may result incontinuous grain boundary precipitation or excessive amounts ofintermetallic phases, nitrides, carbides, or sigma phase, which mayadversely affect the impact and corrosion properties of the alloy, andan article formed from the alloy may be more susceptible tointergranular corrosion.

The exemplary alloys preferably have a microstructure that satisfies thecleanliness requirement both before and after salt bath treatment.Examples of microstructures satisfying the cleanliness requirement areshown in FIGS. 5 and 6. More specifically, FIGS. 5 and 6 show anexemplary alloy having a nitrogen content of approximately 0.14% andformed as a hot-rolled plate followed by heat treating and watercooling. The microstructure of FIG. 5 was heat treated at 1066° C.(1950° F.) for 30 minutes, and the microstructure of FIG. 6 was heattreated at 1093° C. (2000° F.) for 30 minutes. The average grain size ofthe microstructure of FIG. 6 was determined to be about 78 microns. Themicrostructures of FIGS. 5 and 6 are representative of the alloys of thepresent invention.

As shown, there are grain boundaries with no continuous precipitates anda total area fraction of intermetallic phases, nitrides, and carbidesdoes not exceed 1.0% in total area and an area fraction of sigma phasedoes not exceed 0.5%. Microstructural examination was made on a sectiontaken in the longitudinal direction with respect to the direction ofcold work. The examination of the alloys microstructure was carried outin accordance with ASTM E562 with a minimum of 30 fields measured. Themicrostructure had grain boundaries with no continuous precipitates.Intermetallic phases, nitrides, and carbides did not exceed 1.0% intotal area. Sigma phase did not exceed 0.5% in total area.

It is believed that the low nitrogen content of the comparative alloysmay result in the formation of excess grain boundary precipitation ofcarbides and sigma phase that may negate against the requirement for aclean microstructure and may reduce ductility and toughness. Thisproblem is resolved by the addition of high amounts of nitrogen but lessthan the solubility limit.

FIGS. 7-11 show calculated weight percentages of various phases expectedto be present at equilibrium conditions for the following compositionsindicated in Table 7 below, which includes comparative alloy 7A andexemplary alloy 7B.

TABLE 7 ALLOY 7A 7B Al 0.217 0.217 Co 0.4584 0.4584 Cr 22.82 22.82 Cu2.01 2.01 Fe 19.5 19.5 Mn 0.73 0.73 Mo 7.12 7.12 Nb 0.152 0.152 Si 0.1740.174 Ta 0.003 0.003 W 1.07 1.07 C 0.003 0.003 N 0.027 0.14 Ni bal. bal.

As shown in FIGS. 7 and 9, cooling alloys 7A and 7B from a molten stateunder equilibrium conditions is expected to initially result in theformation of a single-phase austenitic gamma phase. At temperatures lessthan about 980° C. (1800° F.), the undesirable sigma phase becomesstable, and at temperatures less than about 650° C. (1200° F.), theundesirable laves phase becomes stable. As shown in FIGS. 8 and 10,small amounts of carbides and nitrides, as well as p-phase, becomestable at lower temperatures in alloys 7A and 7B.

Although the present invention is not limited by theory, it is believedthat, based on a comparison of alloy 7A with alloy 7B, the M6C and M23C6phases may be destabilized for alloy 7B. Likewise the sigma solvus isbeneficially dropped from ˜985° C. (˜1805° F.) for alloy 7A to 960° C.(1760° F.) for alloy 7B. Thus, based on this equilibrium data, it isexpected that alloy 7B may tend to result in reduced precipitation ofcarbides and sigma phase.

Additionally, FIGS. 12 and 13 show Time-Temperature-Transformation (TTT)and Continuous-Cooling-Transformation (CCT) diagrams for alloy 7A, andFIGS. 14 and 15 show TTT and CCT diagrams for alloy 7B. As shown inFIGS. 12 and 13, a sigma nose is calculated to be present at 3.63 hours(TTT) and 3.33 hours (CCT) for alloy 7A, while a sigma nose iscalculated to be present at 6.45 hours (TTT) and 5.92 hours (CCT) foralloy 7B. Thus, a comparison of the sigma noses for alloy 7A with thesigma noses for alloy 7B shows that avoidance of sigma phase would beexpected to be facilitated by using the composition of alloy 7B havingthe higher nitrogen content as long as the temperature and time oftransformation is controlled to be below the ranges shown in FIGS. 14and 15, thus resulting in a clean microstructure.

However, levels of nitrogen exceeding the solubility limit of the alloyduring cooling tend to precipitate nitrides and/or carbonitrides, whichresults in loss of ductility, toughness, and fabricability, negatesagainst a clean microstructure, and also robs the matrix of thestrengthening elements—molybdenum, tungsten, and chromium—therebyreducing the strength and the corrosion resistance of the alloy. Atslightly beyond the solubility limit of nitrogen, a degree ofstrengthening may result from Cr₂N dispersoids within the matrix grains.However, higher amounts of nitrogen contribute to grain boundary filmformation that negates a clean microstructure and contributes to loss ofductility and toughness. Thus, amounts of nitrogen significantly abovethe solubility limit should be avoided.

Corrosion Resistance

The alloy of the present application has good corrosion resistance tosour gas and oil atmospheres as well as resistance tostress-corrosion-cracking and hydrogen embrittlement insulfur-containing H₂S—CO₂—Cl⁻ environments. The alloy preferably hasbroad resistance to sensitization, which causes intergranular corrosionof weld heat affected zones.

Table 8 below shows corrosion data for an exemplary alloy 6B, showingvery low corrosion rates with and without salt bath treatment, thusevidencing the good corrosion resistance.

TABLE 8 Corrosion Results for Comparison of as Cold Worked and as ColdWorked + Salt Bath Exposed Samples of Alloy 6B. Environment: ASTM A262-C(Huey Test) at Boiling for a Period of 240 Hours Corrosion CorrosionCorrosion Average Rate @ Rate @ Rate @ Corrosion Anneal Prior to 48Hours 96 Hours 96 Hours Rate Heat Cold Working (mpy) (mpy) (mpy) (mpy)30% Cold Worked Samples 6B 1950° F./ 9 6 5 7 1 Hr/WQ 35% Cold WorkedSamples 6B 1950° F./ 9 6 5 7 1 Hr/WQ 40% Cold Worked Samples 6B 1950°F./ 10 6 6 7 1 Hr/WQ 45% Cold Worked Samples 6B 1950° F./ 10 5 5 7 1Hr/WQ 60% Cold Worked Samples 6B 1950° F./ 9 5 5 7 1 Hr/WQ Cold Worked +Salt Bath Huey Results 30% Cold Worked Samples + Salt Bath Exposure 6B1950° F./ 10 5 5 7 1 Hr/WQ 35% Cold Worked Samples + Salt Bath Exposure6B 1950° F./ 9 5 5 6 1 Hr/WQ 40% Cold Worked Samples + Salt BathExposure 6B 1950° F./ 9 6 5 7 1 Hr/WQ 45% Cold Worked Samples + SaltBath Exposure 6B 1950° F./ 9 6 5 7 1 Hr/WQ 60% Cold Worked Samples +Salt Bath Exposure 6B 1950° F./ 8 5 5 6 1 Hr/WQ

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

The invention claimed is:
 1. A solid-solution nickel-based alloy for usein sour gas and oil environments, comprising, in percent by weight:chromium: min. of 21.0 and max. of 24.0%; iron: min. of 17.0 and max. of21.0%; molybdenum: min. of 6.5 and max. of 8.0%; copper: min. of 1.0 andmax. of 2.5%; tungsten: min. of 0.1 and max. of 1.5%; sol. nitrogen:min. of 0.08 and max. of 0.20%; manganese: max. of 4.0%; silicon: max.of 1.0%; carbon: max of. 0.015%; aluminum max of. 0.5%; and a totalamount of niobium, titanium, vanadium, tantalum and zirconium: max of0.45%; the balance being nickel and incidental impurities.
 2. The alloyof claim 1, wherein the maximum chromium content is 23.5%.
 3. The alloyof claim 1, wherein the minimum molybdenum content is 6.8%.
 4. The alloyof claim 1, wherein the maximum molybdenum content is 7.8%.
 5. The alloyof claim 1, wherein the minimum copper content is 1.5%.
 6. The alloy ofclaim 1, wherein the minimum tungsten content is 0.50%.
 7. The alloy ofclaim 1, wherein the minimum sol. nitrogen content is 0.10%.
 8. Thealloy of claim 1, wherein the maximum sol. nitrogen content is 0.19%. 9.The alloy of claim 1, wherein a content of Mo+½ W is controlled to be aminimum of 7.6%.
 10. The alloy of claim 1, wherein a content of Mo+½ Wis controlled to be a maximum of 8.5%.
 11. The alloy of claim 1, whereinan overall nitrogen content is a maximum of 0.20%.
 12. The alloy ofclaim 1, wherein the alloy has a low temperature ageability property,Δ(YS×El), of greater than 0 when a yield strength after aging (YS_(aa))is at least 145 ksi, wherein Δ(YS×El) is the 0.2% Offset Yield Strength(ksi) times Percent Elongation after aging (YS_(aa)×El_(aa)) minus the0.2% Offset Yield Strength (ksi) times Percent Elongation before aging(YS_(ba)×El_(ba)).
 13. The alloy of claim 1, wherein a low temperatureageability property satisfies the condition,Δ(YS×El)≦600−(YS_(aa)−145)×20, wherein Δ(YS×El) is the 0.2% Offset YieldStrength (ksi) times Percent Elongation after aging (YS_(aa)×El_(aa))minus the 0.2% Offset Yield Strength (ksi) times Percent Elongationbefore aging (YS_(ba)×El_(ba)).
 14. A method of manufacturing anarticle, comprising: providing a billet formed from the alloy of claim 1that is solution annealed and then cold worked a minimum of 20% to anarticle of predetermined dimensions; and heat treating the cold workedarticle at 468°-537° C. (875°-999° F.) for five minutes to eight hours.15. An article of manufacture formed from an alloy of claim 1, whereinthe article has a cold-worked and aged microstructure, having grainboundaries with no continuous precipitates, wherein upon microstructuralexamination on a section taken in the longitudinal direction withrespect to the direction of cold work, a total area fraction ofintermetallic phases, nitrides, and carbides does not exceed 1.0%, andwherein an area fraction of sigma phase does not exceed 0.5%.
 16. Thearticle of claim 15, wherein the article has a 0.2% offset yieldstrength of at least 1000 MPa (145 ksi) and at least 12% elongation. 17.The article of claim 15, wherein the article has a 0.2% offset yieldstrength of at least 1069 MPa (155 ksi) and at least 10% elongation. 18.The article of claim 15, wherein the article has a 0.2% offset yieldstrength of at least 1138 MPa (165 ksi) and at least 8% elongation. 19.A solid-solution nickel-based alloy for use in sour gas and oilenvironments, comprising, in percent by weight: chromium: min. of 21.0and max. of 23.5%; iron: min. of 17.0 and max. of 21.0%; molybdenum:min. of 6.5 and max. of 8.0%; copper: min. of 1.5 and max. of 2.5%;tungsten: min. of 0.1 and max. of 1.5%; sol. nitrogen: min. of 0.08 andmax. of 0.20%; manganese: max. of 4.0%; silicon: max. of 1.0%; carbon:max of. 0.015%; aluminum max of. 0.5%; and a total amount of niobium,titanium, vanadium, tantalum and zirconium: max of 0.45%; the balancebeing nickel and incidental impurities.